Download 2 Ionic equilibria - University of Basrah

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COLLEGE OF PHARMACY
UNIVERSITY OF BASRAH
Ionic equilibria
By
Dr. Mohammed Sattar
2016/2017
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COLLEGE OF PHARMACY
UNIVERSITY OF BASRAH
Outlines
ØTheories
ØAcid-­base equilibria ØCalculation of pH, acidity constants ØThe effect of ionic strength. 2
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COLLEGE OF PHARMACY
UNIVERSITY OF BASRAH
Theories
Arrhenius Theory
• Arrhenius defined an acid as a substance that liberates hydrogen ions and a base as a substance that supplies hydroxyl ions on dissociation in aqueous media. 3
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Brönsted–Lowry theory
• According to the Brönsted–Lowry theory, an acid is a substance, charged or uncharged, that is capable of donating a proton, and a base is a substance, charged or uncharged, that is capable of accepting a proton from an acid. 4
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• The strength of an acid or a base varies with the solvent.
• HCl is a strong acid but it is a weak acid in glacial acetic acid. • Acetic acid, which is a weak acid, is a strong acid in liquid ammonia.
• Consequently, the strength of an acid depends not only on its ability to give up a proton but also on the ability of the solvent to accept the proton from the acid. T his is called the basic strength of the solvent. • In the Brönsted–Lowry classification, acids and bases may be anions such as HSO4-­ and CH3COO-­, cations such as NH4+ and H3O+, or neutral molecules such as HCl and NH3. Water can act as either an acid or a base and thus is amphiprotic. 5
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Solvent classification
• A protophilic or basic solvent is one that is capable of accepting protons from the solute. Eg. acetone, ether, and liquid ammonia.
• A protogenic solvent is a proton-­donating compound and is represented by acids such as formic acid, acetic acid, sulfuric acid, liquid HCl, and liquid HF. • Amphiprotic solvents act as both proton acceptors and proton donors, and this class includes water and alcohols. • Aprotic solvents, such as the hydrocarbons, neither accept nor donate protons, and, being neutral in this sense, they are useful for studying the reactions of acids and bases free of solvent effects. 6
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Protolytic reactions or protolysis. Acid-­‐base reactions occur when an acid reacts with a base to form a new acid and a new base. So it is the reaction that involve a transfer of a proton
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Lewis Electronic Theory. • According to the Lewis theory, an acid is a molecule or an ion that accepts an electron pair to form a covalent bond.
• A base is a substance that provides the pair of unshared electrons by which the base coordinates with an acid. • Lewis acid eg boron trifluoride
and aluminum chloride,
• Lewis base eg amines, ethers, and carboxylic acid anhydrides, are classified as bases according to the Lewis definition. 8
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Sörensen's pH • The hydrogen ion concentration of a solution varies from approximately 1 in a 1 M solution of a strong acid to about 1 × 10-­14
in a 1 M solution of a strong base • The pH of a solution can be considered in terms of a numeric scale having values from 0 to 14, which expresses in a quantitative way the degree of acidity (7 to 0) and alkalinity (7-­14). • The value 7 at which the hydrogen and hydroxyl ion concentrations are about equal at room temperature is referred to as the neutral point, or neutrality. T he neutral pH at 0°C is 7.47, and at 100°C it is 6.15. 9
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Rate of forward Rate of backward At equilibrium
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At large conc. O f water (diluent)
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Ka= Kw/Kb
Kb =Kw/Ka
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Ionization of Polyprotic Electrolytes.
• Acids that donate a single proton and bases that accept a single proton are called monoprotic electrolytes. • A polyprotic (polybasic) acid is one that is capable of donating two or more protons, and a polyprotic base is capable of accepting two or more protons. • A diprotic (dibasic) acid, such as carbonic acid, ionizes in two stages, and a triprotic (tribasic) acid, such as phosphoric acid, ionizes in three stages. • In any polyprotic electrolyte, the primary protolysis is greatest, and succeeding stages become less complete at any given acid concentration. 14
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Ionization of Polyprotic Electrolytes.
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Ampholytes
Amphoteric compound
Zwitterion
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Isoelectric point (IEP)
is pH at which zwitterion c oncentration e.g. pH of m ilk 6.6 ( casein IEP=4.6)
at maximum
pH=2.48
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Ionic strength 25
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Ionic strength • ionic strength-­-­-­I or μ -­-­-­a measure of the total ion concentration
in solution-­-­-­-­but ions with more charge are counted more due to stronger electrostatic interactions with other ions (I.e., can influence the increase “ionic atmosphere” greater than singly charged ions!) 26
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For z witterion molecule
For monotropic molecule
Kr=Salting in constant=0.32 for
amino acid in water
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0.1
0.1
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Thanks for your attention 30
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